1. Field of the Invention
The present invention relates to electromagnetic interference (EMI) shielding, and more particularly, to the use of multi-layered shielding substrates for optimizing EMI attenuation at frequencies over 100 megahertz (MHz) and especially much higher frequencies in a see-through configuration.
2. Background of the Invention
The atmosphere of industrialized society is constantly bombarded with electromagnetic radiation. Such radiation exists in a wide frequency band and typically emanates from electrical equipment as radio waves. While moderate levels of electromagnetic radiation are currently considered to be relatively harmless to living tissue, this same radiation wreaks havoc in the operation of some electronic instruments and equipment.
Shielded enclosures are required to isolate sensitive apparatus from electromagnetic radiation in a variety of situations, including nuclear magnetic resonance (NMR) imaging, electronic testing of communication equipment, and confidential data communications. Often, emission from electronic equipment also needs to be contained. EMI shielding is often accomplished by coating the walls of enclosures with one or more electrically continuous layers of a metallic conducting material (copper, aluminum, bronze, or steel for instance). The shielding layers are usually grounded at a single point to conduct to the earth the electromagnetic energy absorbed by the shield. It is of crucial importance that there be no gaps in this electrically conducting shielding even where apertures such as doors and windows are provided.
Often windows are required in a variety of shielding enclosures so that observers, supervisors, and even operating technicians may remain outside the enclosure while visually monitoring activities occurring inside the enclosure. In NMR imaging, also known as Magnetic Resonance Imaging (MRI), the patient and the extremely sensitive MRI equipment are located inside an EMI shielded enclosure. This is to create an interference free environment to avoid artifacts or pictorial flaws in the final image. Typical examples of screens and windows designed into EMI attenuation devices are disclosed in U.S. Pat. Nos. 4,701,801; 5,012,041; 5,017,419; 5,239,125; and 5,295,046.
Windows for EMI/RFI enclosures must be shielded and in continuous electrical contact with the wall shielding of the enclosure. Also, the window substrate must provide enough xe2x80x9csee-throughxe2x80x9d and in some cases xe2x80x9chear-throughxe2x80x9d transparency to allow for outside monitoring by supervising personnel. As such, it is desirable to minimize optical distortion by the window substrate.
Windows comprising single and two layered screens, optically clear metallized coated-glass or clear plastic or other conductive materials, or combinations of the above mentioned, have been utilized to attenuate EMI while being optically transmissive. Shielding effectiveness is dependent upon parameters that include screen mesh patterns, wire diameter, grid count, and conductive material thickness and type.
When screens are utilized, one determinant of shielding effectiveness (SE) is a function of the distance (g) between the grid wires of each individual screen, restated in the following equation:
SE=f(g)
As gxe2x86x920, the E-field attenuation increases at higher frequencies. However, this also results in an unwanted decrease in desirable xe2x80x9csee-throughxe2x80x9d and xe2x80x9chear-throughxe2x80x9d characteristics of the resulting substrate.
Windows comprised of one screen provide some EMI attenuation, but this attenuation decreases by approximately 20 dB for each power of ten (xe2x80x9cdecadexe2x80x9d) increase in frequency above 1 MHz, as illustrated in White, Donald R. J. Electromagnetic Shielding Materials and Performance. 2nd Ed., 1980 (Don White Consultants, Inc., Gainesville, Va.). As a result, appreciable amounts of electromagnetic radiation permeate through single screen structures as frequencies increase appreciably.
Windows consisting of two parallel screens improve attenuation somewhat, as illustrated in the exemplary two-screen EMI window disclosed in U.S. Pat. No. 5,012,041, and incorporated herein by reference. The ""041 patent teaches using two parallel screens comprising relatively different wire sizes and spacings to both attenuate EMI and also reduce moire patterns.
However, the inventors have found that a double parallel screen structure produces attenuation performance anomalies above 100 MHz and especially at frequencies in the one giga Hertz (GHz) to 10 GHz frequency range. These anomalies occur as a function of the screen separation and are especially severe whenever the distance separating the two planar screens equals a multiple of one half the wavelength (xcex) of the impinging electromagnetic wave, as defined by the equation c=fxcex where c is the speed of light and f is the frequency of the wave. In these instances the space between the screens forms a resonant cavity at each of the corresponding frequencies and integral multiples thereof. This results in a significant degradation in EMI attenuation at or near the resonant frequency and its harmonics. The existence of such resonant transmission poses problems for imagers, one such problem being that the resulting transmitted radiation may be mistaken for a signal emanating from an object under study. Furthermore, the inventors have found that resonant transmission can occur at other frequencies. As a result of such resonance-generating phenomena, two-screen systems often have poorer attenuation characteristics at certain frequencies compared to single screen systems.
Simply adding additional layers of screening to the two-screen configuration without regard to the relative separation between the screens can be counter-productive because the additional screens introduce additional resonant frequencies which are a function of the spacing between the screens.
A need exists in the art for a method and device that provides improved attenuation of EMI at frequencies higher than 10 MHz and especially in the 1 GHz to 10 GHz range while preserving sufficient see-through and hear-through characteristics. The method and device should utilize commercially available materials and should also facilitate easy maintenance and replacement.
An object of the present invention is to provide an EMI shielding method and device having optimum attenuation performance that overcome many of the disadvantages of the prior art.
Another object of the present invention is to provide a method for attenuating EMI at a myriad of frequencies. A feature of the method is the juxtaposition of the surfaces of light-weight see-through and hear-through electrically conductive materials in a predetermined configuration relative to each other so that a first series of the materials attenuates incoming EMI and a second series of materials further attenuates the attenuated EMI. An advantage of the invention is a reduction of the propagation of resonant electromagnetic radiation through multiple layers of materials so as to tune or optimize performance of the method at specific electromagnetic radiation frequencies above 10 MHz.
Another object of the present invention is to provide a device with EMI shielding performance at or exceeding 100 decibel (dB) attenuation levels for frequencies from 100 MHz up to 10 GHz. A feature of the present invention is the incorporation of a plurality of screens comprised of conductive materials. An advantage of the present invention is the graduated attainment of higher levels of EMI/RFI shielding performance in a see-through and/or hear-through environment.
Still another object of the present invention is to provide an EMI attenuating device comprised of lightweight screens or other conductive but optically transparent materials. A feature of the invention is the specific juxtaposition of the surfaces of the screens relative to each other. An advantage of the invention is the incorporation of cavities which stymie radiation-wave resonance and harmonics at EMI frequencies of interest, therefore reducing the propagation of the EMI through the substrate.
Briefly, the invention provides for an EMI attenuation device comprising a first electrically conductive member spatially positioned adjacent to a second electrically conductive member; a third electrically conductive member spatially positioned adjacent to said second electrically conductive member and with said first electrically conductive member being spatially separated from said second electrically conductive member a distance dissimilar to the distance spatially separating said second electrically conductive member from said third electrically conductive member.
The invention also provides a method for reducing the transmission of electromagnetic radiation comprising subjecting the electromagnetic radiation to regions which prevent passage of the electromagnetic radiation and which also prevents the production and passage of resonant frequencies of the electromagnetic radiation.
Also provided is a method for attenuating electromagnetic radiation comprising subjecting the radiation to a plurality of resonant cavities having dissimilar dimensions and different resonant frequencies so that radiation resonating within a specific cavity is blocked by said other cavities.
A window for use in conjunction with an EMI-shielded enclosure is provided with the window comprising at least two electrically conductive surfaces with no two adjacent surfaces being parallel.
This invention further provides a method for attenuating electromagnetic radiation comprising causing the electromagnetic radiation to strike a first electrically conductive substrate, resulting in a first portion of the electromagnetic radiation not permeating or passing through the first substrate and a second portion of the electromagnetic radiation permeating through the first substrate; and subjecting the now-permeated radiation to a means for preventing the production of resonance frequencies of the now-permeated radiation.
In another embodiment, the invention provides for a device for attenuating electromagnetic interference comprising a first defined space having a plurality of walls configured from first and second electrically conductive shielding means; a second defined space having a plurality of walls configured from said second electrically conductive shielding means and a third electrically conductive shielding means; and means for positioning said first, second and third shielding means whereby distances between adjacent walls of said first and second shielding means are dissimilar to corresponding collinear distances between adjacent walls of said second and third shielding means.
Still another embodiment of the invention is an EMI attenuation device comprising two electrically conductive surfaces with said surfaces separated by an average distance, wherein said average distance is selected to obtain maximum attenuation at a pre-determined frequency.